1. Field of the Invention
The present invention is directed to a vanadium-containing catalyst system useful in the polymerization of olefins. More particularly, the present invention is directed to a vanadium-containing catalyst system particularly adapted to the formation of olefin polymers used in blow molded articles.
2. Background of the Prior Art
The use of olefin polymers in blow molded articles is well established in the art. Olefin polymers combine relative low cost with chemical inertness. As such, they are particularly suitable for use as containers. Containers formed of olefin polymers, as those skilled in the art are aware, are usually prepared by blow molding techniques. Blow molded polyolefin bottles are often employed as inert containers for liquids whose properties but for the inertness of the polyolefin container would be affected by the material of construction of the container. As a result, foods, soaps, detergents and the like, which cannot be affected by the material of construction of the container, are oftentimes vended in polyolefin containers. Obviously, breakage, a problem associated with glass, is not a detriment of polyolefin containers, which otherwise meets this requirement of chemical inertness, emphasizes its desirability over other materials used in such applications.
Those skilled in the art are particularly aware of the preferred suitability, among other olefin polymers, of ethylene polymers in the formation of articles formed by blow molding techniques. Ethylene polymers, either homopolymers or copolymers wherein the comonomer is a higher .alpha.-olefin, such as butene or hexene, in a low concentration, having a preferred density in the range of between about 0.94 and about 0.97, are particularly suitable, because of their low cost, chemical inertness and non-brittleness, for use as milk bottles, detergent bottles and the like.
Those skilled in the art of blow molding plastic articles are aware of the essential combination of processing and property characteristics required to successfully prepare blow molded articles. Indeed, the continuing development of new olefin polymers, especially ethylene polymers, has focused upon the development of resins having both processing and article physical properties required in high volume production of high quality containers and other blow molded articles.
Although the criteria for acceptable plastic containers and bottles are well established in the art, the criteria for acceptability of olefin polymers in terms of its processability varies with the equipment employed. Two classes of blow molding apparatus are utilized in the formation of blow molded polymeric articles. The first of these classes is continuous blow molding apparatus. Suffice it to say, the processing speed of this type of blow molding apparatus is inversely proportional to the viscosity of the molten polymer processed therein. A commonly employed measure of the suitability of a resin for continuous blow molding operations is the property known as .eta.*.sub.100. .eta.*.sub.100 measures the viscosity, in poise, of a polymeric resin rotated at 100 reciprocal seconds. The lower the viscosity of a polymer, the easier and more efficient is its processability during blow molding fabrication, on continuous blow molding apparatus, into blow molded articles.
The processability properties of a plastic resin fabricated into blow molded articles are more complex than its product properties. This is because of the common utilization of a second class of blow molding apparatus in the formation of blow molded articles. In addition to continuous blow molding equipment, this second class of blow molding apparatus utilizes intermittent reciprocal screws. This non-continuous apparatus, although dependent upon polymer viscosity, is more critically dependent upon another polymer rheological property, zero land die swell.
Three physical properties which mark a good blow molded article are environmental stress crack resistance (ESCR), top load ESCR and column crush. For a polymer to be successfully employed in blow molded article applications these properties must be sufficiently high to insure requisite bottle leak proofness and strength.
An additional advantage of catalyst systems which produce superior quality resins relates to the increasing practice in this art of blending recycled polymer with virgin polymer. As those skilled in the art are aware, recycled polymer possesses inherently inferior physical properties compared to virgin polymer. This results in a blend of virgin and recycled polymer having inferior properties compared to virgin polymer alone. Therefore, the blending, with a recycled polymer, of a virgin polymer having the enhanced properties produced by an improved catalyst system provides a resin composition which, when molded, produce molded articles having improved properties compared to those blended from recycled polymer and the virgin polymers of the prior art.
To meet these requirements the vast majority of olefin polymers destined for processing on blow molding apparatus are presently synthesized into high density polyethylene (HDPE) using silica supported chromium catalyst systems. Such systems produce HDPE having broad molecular weight distribution.
Chromium-based catalyst systems, commonly employed to prepare ethylene polymers used in the manufacture of blow molded articles, especially bottles and the like, suffer from a major failing complicating the polymer-forming reaction. That is, chromium-based catalysts are very sensitive to polymerization upset caused by even very minor ethylene feedstock contamination. This is especially critical in that even minute concentrations of oxygen or moisture disrupt the process of polymerizing ethylene polymers when catalyzed by chromium-based catalysts, which, as stated above, is the standard prior art catalyst used in the polymerization of blow moldable grades of ethylene polymers.
Traditional Ziegler-Natta catalyst systems, based on the use of the single transition metal titanium, provide excellent control of the molecular weight of product olefin polymer albeit they do not adequately control molecular weight distribution. Thus, although these traditional titanium-containing catalyst systems have been employed in this application, they have been used in costly multiple reactor polymerizations. Such reaction schemes overcome the molecular weight distribution problem but at no small cost.
Multiple reactor polymerizations provide reduced production rates, increased process complexity, significant gelling, which gelling results from the immiscibility of the resin product of a first reactor with the resin product of a second reactor in the liquid state, increased capital expense and increased operating costs.
More recently, dual-site or multi-site Ziegler-Natta catalyst systems, which meet the requirements of both absolute molecular weight as well as molecular weight distribution, have been developed to provide requisite product and processability polymer physical properties.
One such development is set forth in U.S. Pat. No. 4,918,038 to Samuels et al. which describes a catalyst system which comprises a vanadium-containing component. The vanadium-containing component is the reaction product of a vanadium halide and a boron trihalide or an aluminum halide which may include up to two alkyl groups. The four-component catalyst system of the '038 patent includes, as a second component, a zirconium-magnesium halogen-containing complex or a vanadium oxy compound. The third and fourth components of the '038 catalyst system are identical with the second and third catalyst components of the aforementioned '853 patent.
Other developments pertinent to the subject of the instant invention include U.S. Pat. No. 4,004,071 to Aishima et al. which describes a method for homopolymerizing ethylene or copolymerizing ethylene and another olefin wherein the catalyst utilized is the reaction product of a hydrocarbon-soluble complex containing aluminum and magnesium and at least one compound selected from the group consisting of titanium and vanadium compounds which contain at least one halogen atom. These catalyst systems include a trialkyl aluminum or dialkylaluminum hydride cocatalyst.
U.S. Pat. No. 4,378,304 to Dombro describes a catalyst component, which, in conjunction with a cocatalyst and hydrogen, polymerizes ethylene. This catalyst is the sequential reaction product of (1) a porous support of a Group IIA organometallic compound; (2) water or a hydrocarbyl alcohol; and (3) a Group IVB and/or Group VB transition metal compound. The cocatalyst utilized with the catalyst component is a Group IA, IIA, IIIA and/or IIB organometallic compound. It is emphasized that the preferred transition metal compound of the catalyst component is a titanium-containing compound.
U.S. Pat. No. 4,402,861 to Hoff describes an olefin polymerization catalyst which includes a solid catalyst component formed by reacting, in the presence of a solvent, any one of silica, alumina and silica-alumina, preheated to between 200.degree. C. and 900.degree. C., with an alcohol and a magnesium alkyl or a magnesium-aluminum complex. The sequence of reaction of the alcohol and the organomagnesium compound is optional. That is, either the alcohol or the organomagnesium compound can contact the solid inorganic oxide first followed by contact with the second of the two compounds. Finally, the resultant product is contacted with a titanium, vanadium or zirconium halide, oxyhalide or alkoxyhalide.
U.S. Pat. No. 4,426,317 to Rogers describes a catalyst composition useful in the polymerization of olefins wherein an inorganic oxide, predried to remove adsorbed water, is reacted with an organometallic compound of a Group III metal, preferably a trialkylaluminum compound. That product, in turn, is contacted with a vanadium compound wherein the vanadium has a valence of at least +3.
U.S. Pat. Nos. 4,434,242 to Roling et al. and 4,435,518 to Pennington describe similar olefin polymerization catalyst compositions. Both of these catalyst compositions are obtained by drying an inorganic oxide having surface hydroxyl groups to remove adsorbed water. The so-treated oxide is contacted with an organometallic compound having at least one alkyl group attached to a Group III metal. This organometallic compound is preferably a trialkylaluminum. The thus-treated product is contacted with a vanadium halide and finally contacted with, in the case of the '242 patent, an ether-alcohol and, in the case of the '518 patent, an alcohol.
U.S. Pat. No. 4,435,520 to Aylward describes a catalyst composition similar to the aforementioned U.S. Pat. Nos. 4,434,242 and 4,435,518 but for the omission of the final alcohol or ether-alcohol contacting step. However, this patent requires that the vanadium halide be a mixture of vanadyl chloride and vanadium tetrachloride.
U.S. Pat. No. 5,334,567 to Menon et al. teaches an .alpha.-olefin polymerization catalyst system. The system includes a solid catalyst component formed by admixing a solid inorganic oxide, a Group III organometallic compound, a vanadium-containing compound and an organic reagent which may be an alcohol, an ether, a carbonate, an amine, a phosphorus compound or a dialcohol. The catalyst system includes a halosilane cocatalyst and may include a modifying compound having the formula M(R.sup.4).sub.d X.sub.3-d where M is aluminum or boron; X.sup.3 is halogen; R.sup.4 is saturated hydrocarbon having 1 to 12 carbon atoms; and d is 0 or an integer of 1 to 3.
In addition to the above patents, U.S. Pat. No. 4,892,853 to Cann et al., describes a vanadium catalyst system which includes a vanadium-containing catalyst component. That component is the reaction product of a vanadium halide or a vanadium acetylacetonate and an electron donor which is a liquid, organic Lewis base. That base may be an ester, a ketone, an amine, an alcohol, an ether or mixtures thereof. The catalyst composition of the '853 patent includes a hydrocarbyl aluminum cocatalyst and a halocarbon promoter.
Although the above discussion of the prior art barely skims the surface of catalyst systems employed in the synthesis of blow molding resins, it is apparent that the catalyst systems of these and other disclosures, employed to produce an olefinic, especially ethylenic, polymer having the required combination of product and process characteristics necessary for the successful production of blow molded articles, especially bottles and other containers of liquid foods, detergents and the like, can be improved to provide properties better suited to this application.